1. Field of the Invention
The present invention relates to compositions and methods for forming multilayer ceramic electronic components from flexible glass-ceramic articles termed greensheets and, in particular, to such electronic components having enhanced strength and mechanical reliability.
2. Description of Related Art Ceramics have found widespread use in electronics as a substrate for integrated circuit packages. Metallized circuit patterns are applied to the ceramic substrate in the form of a greensheet, and ceramic and metallization are cosintered to create a monolith of substrate and circuitry. Multi-layer ceramic (MLC) circuit packages are constructed by combining ceramic particles and organic binders into unfired, or "greensheet", tape. Inter-layer electrically conductive paths, known as "vias", are then inserted (punched) through the greensheet tape and filled with a conductive metal paste, forming electrical interconnections between the circuits on each tape layer. The metallized layers are then stacked and processed to form the monolith MLC electronic component with three-dimensional circuitry.
The ceramic substrate can be made from many types of oxide and nonoxide materials. These include glasses, glasses mixed with crystalline oxides and nonoxides (referred to herein as glass plus ceramic) as well as glasses that crystallize during processing to form glass-ceramics.
The mechanical reliability of ceramic chip carriers is often related to the strength of the ceramic. Typical glass bonded alumina substrates have flexural strengths near 345 Mpa (50 K psi). These are among the highest strength materials. A disadvantage of alumna substrates made with substantial amounts of alumina is the high dielectric constant that is undesirable for high speed chip carrier packaging. Chip carriers made with high glass or glass-ceramic content can have lower dielectric constant values and are thus more desirable. Glass-ceramic substrates are often subjected to the same types of stresses encountered by alumina substrates during pin attach and flexural bending during Land Grid Array (LGA) socket insertions. The flexural strength of typical glass-ceramics is about 228 Mpa (35 K psi). To improve the reliability and expand the applications for glass-ceramic substrates, there is a need for glass-ceramics to have increased strength without increasing the complexity of manufacture or changing the composition of the MLC build process. In addition, the flexural modulus of glass-ceramic ideally is controllable to allow the lower modulus of glass to be exploited for greater second level attach reliability. These more flexible substrates can endure the motion of printed circuit boards to which they are joined without causing cracking at the solder connections.
To further increase the usefulness of this increased strength, glass-ceramic chip carriers produced using an XY constraining force that controls shrinkage during sintering are desirable. The casting of suspensions of ceramic, glass and glass-ceramic materials to form layers or sheets which are punched, patterned, layered and then sintered to produce a ceramic or glass-ceramic multilayer substrate material is known in the art. The doctor blade method is one method for producing a glass-ceramic greensheet. Typically, the glass and ceramic powders are mixed with an organic solvent, a plasticizer and a binder forming a slurry, the slurry is cast in a regulated thickness on a carrier film with the aid of a doctor blade, and the applied layer of the slurry is then dried. The glass is typically silica, the ceramic is typically alumina and a butyral type resin like polyvinyl butyral is the binder. A cellulose type resin like ethyl cellulose or polyvinyl alcohol may also be used as the binder.
Alumina and glass are typically used as the ceramic component of the greensheet for both the insulating layers and dielectric layers. The laminated greensheets are typically sintered or fired at a temperature of about 1,500.degree. C. or higher. In view of the high sintering temperature, the vias and wiring must be made of molybdenum, tungsten, or a like conductor material having a high melting point.
There is a need, however, for MLC's which may be sintered at a lower temperature such as 800 to 1000.degree. C. so that a more electrically conductive metal with a lower sintering temperature such as copper can be used. Attempts to increase the strength of ceramic substrates made using glass mixed with ceramic powders have included the use of a compressive layer on either or both the top and bottom layers of the MLC. U.S. Pat. Nos. 3,911,188; 4,506,024; 5,047,374; and 5,411,563 are exemplary and are incorporated herein by reference. When using surface compressive layers, the surface layer is chosen so as to have a coefficient of thermal expansion (CTE) that is lower than that of the underlying layers after sintering. This is typically performed by changing the composition of the surface layers or the ratio of the glass and ceramic components used in these greensheets.
The previously cited prior art includes processing by post sinter strengthening such as ion exchange or surface coating. These cannot easily be used for making substrates to be used as chip carriers due to interactions of these processes or coatings with surface metallization. Most desirable is a single sintering step which will produce high strength substrates with no further processing.
U.S. Pat. No. 5,411,563 references the use of glass powders in the range of 0-100% as being suitable for making precursory substrates (greensheets) for use in strengthened ceramic/glass substrates. Glass has a very low strength owing particularly to surface flaws induced during processing or environmental attack. One hundred percent glass substrates would have little or no usefulness as chip carriers due to the low strength and fracture toughness. Examples cited wherein additions to the glasses by ceramic powders improve the strength to approximately 250 Mpa when using strengthening outer layers may still not be suitable for applications where alumina-like strengths are needed.
Likewise, the presence of residual glass in the fired substrate can lead to susceptibility to chemical attack since glasses are known to be less resistant to corrosion than their crystalline counterparts.
In addition, the presence of glass after sintering can be undesirable where substrates in use may be subjected to elevated temperatures which can cause softening and deformation of the substrate.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a glass-ceramic greensheet casting composition which provides greensheets for use as top and/or bottom layers on a multilayer glass-ceramic substrate to increase the strength and flexibility properties of the formed MLC substrate electronic component.
It is another object of the present invention to provide a glass-ceramic greensheet having enhanced strength and flexibility properties when sintered as a top and/or bottom layer of a multilayer glass-ceramic substrate.
A further object of the invention is to provide a process for producing a glass-ceramic substrate greensheet wherein the greensheet increases the strength and flexibility properties of a glass-ceramic multilayer substrate when used as the top and/or bottom layer of the substrate.
It is yet another object of the present invention to provide a process for fabricating a multilayer glass-ceramic substrate from greensheets made using the methods and composition of the invention.
Another object of the present invention is to provide greensheets and multilayer glass-ceramic substrates made from the greensheets using the compositions and/or processes of the invention.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.